Which reaction is easier to artificially create: fusion or fission?
Fission. Reasons below...
To create fission, one needs only collect an amount of an isotope such as uranium-235, which has an unstable nucleus. Reactors that use these fissile materials have become very commonplace, being used in 31 countries to produce 11% of the world's energy demand (according to the World Nuclear Organization). Read about it here:
Nuclear fusion, on the other hand requires that one first create a plasma of hydrogen (or other light nuclei) at enormously high temperatures. Then there is the problem of how to contain matter at such temperature - once you have made it, what do you keep it in?
Science is gradually solving these problems, but only recently have they managed to create a demonstration of fusion in which more energy was released by the reaction than had to be put into the process which created the reaction. And this is still a long way from a working fusion reactor.
So, from a practical point of view, we have been successfully using fission for many years, but because of its many inherent problems, the building of a workable fusion reactor appears to still be a long way off.